One existing scanning technique employs a resonant cantilever in order to achieve the high frequency scanning of an optical fibre tip, as a compact alternative to resonant mirror/galvanometer scanners. The high frequency or X scan is then combined with a slow or Y scan to produce a standard raster scanning pattern. The slow scan is usually not resonant and can have a sawtooth function, with relatively rapid retrace.
In such an arrangement, mechanical considerations require a resonant sinusoidal spot motion in the X or fast direction of the scan. Although a TV raster in both directions is more desirable (for optical dosage control and data gathering), a sawtooth scan at constant speed is feasible in only the slow or Y direction. As shown in FIG. 12, under practical conditions about half the scan area is available for data acquisition as indicated by the solid curves for a typical square image area. It is a matter of simple geometry to calculate that, at the end of the solid curves, the spot speed has fallen to 87% of the peak spot speed. The dotted portion of the scan, where spot speed is less than 87% of this maximum, is discarded. The 87% figure is derived from the 87% value for the maximum derivative of the cosine value. This value is somewhat arbitrary, but provides a basis for comparison between different scan patterns. The choice of 87% originates from the inventors' use of half of the amplitude of the sine wave for the raster scan, which corresponds to the ≧87% of maximum speed section of the raster pattern. This is also regarded, based on the inventors' experience, as the acceptable region in terms of image quality without undue distortion.
Typically in existing systems, the Y mechanism is in tandem with the X deflection system and has similar length. However, as demand increases for ever more compact scanners there is a need to develop a combined XY scanner of shortest possible length. If the fibre can itself be deflected in both X and Y directions, as a symmetrical cantilever, the scanner is more compact. The problem is that for practical forces only resonant operation is feasible. For this reason modulated circular patterns have been developed, such as is disclosed in U.S. Pat. No. 6,294,775 (Seibel and Furness).
U.S. Pat. No. 6,294,775 discloses a system in which a fibre tip is typically moved in a circle or ellipse, the radius of which is then modulated so that an area is progressively scanned. Suitably phased X and Y drives can produce the circular motion, effectively resonant in both X and Y directions. However, the modulation of the radius of the scanning circle or ellipse results in a large variation in scanning speed, and a singularity at the centre of the field. This scan pattern is very different from a raster scan, so the resulting circular pattern requires image processing, creating interface problems with standard systems. One pattern that can be produced with the system of U.S. Pat. No. 6,294,775 is shown schematically in FIG. 13. Only that portion of the scan shown with a solid curve would be used for imaging. The portion of the scan shown with a dotted curve corresponds to a spot speed of less than 87% of the peak scan speed and is discarded. Hence most of the central area cannot be accessed. The solid curve again corresponds to a spot speed of ≧87% of the peak spot speed which, in this spiral scan, corresponds to a radius of 87% of the maximum radius.